You're contradicting yourself here - players interested in powerplay modules would play Powerplay simply to get at the modules.
Also - doesn't matter if Powerplay is moved to Open only - preventing game clients connecting will be a thing and there's nothing Frontier can do about that other than completely rewriting the game client and back-end servers into a purely client-server based game.
Players who only grind the modules have no interest in PP and the system and will not participate in any meaningful ways anyways. Hence we have the closest-system-overforitifcation triggers for each power as it's the fastest way to grind the required merits.
That said, this can be done in Open too and has less to no risk.
Players who support a power long term, however, are not primarily interested in the PP specific module, but in the success of said power, meaning he/she will stay loyal for a longer period of time and contributing to strategic goals of the power.
The first group is excluded fro mthis discussion as they do not play any meaningful role. If you only want a specific module from that power, you don't even have the problem to get hunted by PvPers as you are no strategic target and thus, open is perfectly safe.
The second group of players, however, should be visible to hostile forces as their loss is their enemy's reward and by all means far more effective than NPC farming.